The present invention relates to a process for directly reducing ore containing iron oxide in a rotary kiln using a solid, carbonaceous material, such as coal, as both fuel and reductant and, more particularly, to a method and means for recycling char to the kiln in an improved manner.
Many different methods have been suggested and used for carrying out the direct reduction of ores containing iron oxide using carbonaceous materials, particularly coal, as both the heating agent and reductant in a rotary kiln. For example, in some of these processes the coal is fed into the kiln through the discharge end by mechanical or pneumatic means, such as respectively disclosed in U.S. Pat. No. 3,113,859 to Moklebust and U.S. Pat. No. 3,505,060 to Heitmann, and in some it is fed at the center of or along the kiln, such as disclosed in U.S. Pat. No. 3,206,299 to Senior et al. However, considerable problems have been encountered with these various approaches, many of which problems have been overcome by the process disclosed in U.S. Pat. No. 3,890,138 to Hockin. In this latter process a portion of the coal is injected or blown from the discharge end of the kiln, and the remaining portion of the coal is fed with the ore at the charge feed end. The kiln is generally divided into a reducing zone toward the discharge end and a preheat zone toward the feed end, and the coal is distributed in the kiln in such a manner that the amount of coal injected from the discharge end is sufficient to aid in controlling the temperature profile throughout both zones of the kiln.
Although the process of Hockin was developed particularly for use in reducing ilmenite, it has been found that the dual end coal feeding technique improves upon the other coal feeding methods used in reducing iron ore to sponge iron in rotary kilns with the direct reduction process, and the present invention relates to improvements in this latter process when dual end coal feeding is used.
It has been the practice in the art when directly reducing iron ores to sponge iron in a rotary kiln using coal as the reductant, to recover and recycle the charred coal found in the discharge product. This charred coal or char is separated from the metallized portion of the product and collected in a char recycle bin from which it is fed at a selected rate into the feed end of the kiln along with the coal and ore. A problem is properly feeding the char at the feed end of the kiln is presented by a combination of the low bulk density of specific gravity of the char particles and the considerable velocity of the exhaust gases from the kiln which may range in magnitude from 10 to 50 feet per second. Under these conditions much of the light and small particle size recycle char will be carried off into the waste gas system, and although again recovered and recycled from the waste gas, much of the char may be continuously reintroduced at the feed end, ground down, and eventually lost from the process into the waste gas system. Of course, the more char that can be effectively recycled, the less coal need be supplied to run the process, and thus char recycling is important to process efficiency.
The present invention describes an improvement in the process of this general type, particularly directed to reducing ore containing iron oxide, wherein the char recovered from the discharge product is recycled to the kiln by being injected into the discharge end of the kiln with or without blown coal.